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  1. #1

    A theoretical question about condensers

    Hi all,

    I am new to this site, so thank you for taking the time to read my post. I have a question on the fundamentals of the vapour compression refrigeration system that hopefully someone can help me with. I'm sure the answer is straightforward but unfortunately all the literature I have found on the internet doesn't seem to answer my question.

    In a typical one stage compression cycle, the expansion valve is controlled to regulate the low pressure in the evaporator, whether its a thermal expansion valve or electrically controlled valve it will act on the superheat somehow to optimise the evaporating temperature dependent on the duty (or load). Following the evaporator the vapour is compressed but what I don't understand is what sets the pressure/condensing temperature in the high pressure side?

    Is it determined by the temperature of the cooling medium, i.e. ambient air or water? Or is it determined by the compressor?

    And finally, if I am using two cooling mediums at different temperatures to provide all the necessary condensation (i.e. some of the refrigerant is condensed in an air cooler and some in a water cooled condenser - either before or after the air cooler), what sets the condensing temperature then? The pressure will be equal throughout the high pressure side (neglecting pressure drops), so the condensing temperature will also be equal throughout.

    Many thanks again.

  2. #2
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    Well, since it sounds like you're a student, we'll start with an ideal or a perfectly operating system.

    Nothing is really determined by ONE factor in isolation, but rather by a confluence of multiple factors acting in concert.

    Of course, temperature of the condensing medium is a factor, but size of the condenser is also a factor. This factor is typically fixed by the design of the equipment, but it's a factor nonetheless. You'll see this in working on older equipment (with typically smaller condensers) vs newer equipment (with typically larger condensers)

    System load is also a factor (higher load, higher pressures)


    I'll caution you that you're incorrect on this as well:
    the expansion valve is controlled to regulate the low pressure in the evaporator, whether its a thermal expansion valve or electrically controlled valve it will act on the superheat somehow to optimise the evaporating temperature dependent on the duty (or load)
    While you're correct that the metering device controls superheat, that is ALL it controls. Evaporating temperature is also 'set' similarly to the condensing temperature.

  3. #3
    Thanks jpsmith1cm, some good pointers. Whilst I am not a student, I am interested in an ideal cycle yes.

    I'm really interested in what happens if I replace some of the condenser load, that is already satisfied with an air fan, with a water cooled condenser (i.e. turn the fan speed down on the air cooled condenser and install a water cooled condenser after it). The water is cooler than the air, but will this affect the condensing temperature? The load remains unchanged.

  4. #4
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    Is this just a theoretical question or what are we trying to get to? Why on earth would you want to do that?

  5. #5
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    Quote Originally Posted by gateeight View Post
    Thanks jpsmith1cm, some good pointers. Whilst I am not a student, I am interested in an ideal cycle yes.

    I'm really interested in what happens if I replace some of the condenser load, that is already satisfied with an air fan, with a water cooled condenser (i.e. turn the fan speed down on the air cooled condenser and install a water cooled condenser after it). The water is cooler than the air, but will this affect the condensing temperature? The load remains unchanged.
    If it's done properly, it doesn't matter if you use water, air, or both. In Chicago there are still refrigeration condensing units that utiize both air and water.
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  6. #6
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    You see this from time to time as a heat reclaim set up.

  7. #7
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    Quote Originally Posted by ryan1088 View Post
    Is this just a theoretical question or what are we trying to get to? Why on earth would you want to do that?
    I can think of a lot of large package units near dirty processes that we have considered this. Add a small water cooler after the cond. that starts flowing at x head pressure.

  8. #8
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    Combination air & water cooled condensing units were fairly common about 30-40 years ago. They were often used in areas where the ambient air temperatures could get rather high and water availability was limited. They also used them as "water savers" in areas where the cost of city water became expensive.

    They were typically arranged so the compressor discharge gas first went to the air-cooled condenser and then through a water-cooled condenser piped in series with a pressure actuated water regulating valve on the water leaving of the condenser. The regulating valve was set to limit the condensing pressure, so during cooler weather very little water was needed.

  9. #9
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    Quote Originally Posted by gateeight View Post
    Hi all,

    ...Following the evaporator the vapour is compressed but what I don't understand is what sets the pressure/condensing temperature in the high pressure side?

    Is it determined by the temperature of the cooling medium, i.e. ambient air or water? Or is it determined by the compressor?

    And finally, if I am using two cooling mediums at different temperatures to provide all the necessary condensation (i.e. some of the refrigerant is condensed in an air cooler and some in a water cooled condenser - either before or after the air cooler), what sets the condensing temperature then? The pressure will be equal throughout the high pressure side (neglecting pressure drops), so the condensing temperature will also be equal throughout...
    The temperature of the cooling medium is only one factor. The others would be the condenser's capacity (ability to reject heat) and the amount of heat which must be rejected.

    The condensing pressure that results is a balance of all three factors. The condenser capacity is typically expressed in terms of heat rejection over a temperature difference...ie, Btuh/F TD.

    The heat is called the total heat of rejection (THR) and is calculated by adding the refrigerating capacity (Btuh) and the power input (Watts expressed as Btuh) to the compressor.

    The TD is the condensing temperature minus the ambient temperature, which for air cooled condensers is usually between 10F-30F where the niminal ambient temperature is 90F and is defined at the time of equipment is selected. For water-cooled, the standard is 85F for cooling tower water and 75F for city water...giving you either a 10F TD or a 20F TD respectively.

    Let's say you have a system with a one ton load (12,000 Btuh) and the compressor's power input is 880 watts (3000 Btuh). The THR would then be 15,000 Btuh. Your condenser's capacity is 500 Btuh/F TD, so the TD would be 15,000/500 = 30F. If the ambient is 90F, then the condensing temperature would be 90+30 = 120F. You would do a similar calculation for a water-cooled condenser.

    If you were to set up a air & water cooled system, then the one with the lower calculated condensing temperature would determine the condensing pressure...assuming that condenser were capable of handling the load.

  10. #10
    Quote Originally Posted by icemeister View Post
    Let's say you have a system with a one ton load (12,000 Btuh) and the compressor's power input is 880 watts (3000 Btuh). The THR would then be 15,000 Btuh. Your condenser's capacity is 500 Btuh/F TD, so the TD would be 15,000/500 = 30F. If the ambient is 90F, then the condensing temperature would be 90+30 = 120F. You would do a similar calculation for a water-cooled condenser.

    If you were to set up a air & water cooled system, then the one with the lower calculated condensing temperature would determine the condensing pressure...assuming that condenser were capable of handling the load.
    icemeister, thank you very much for your detailed response. Your final statement is exactly the answer I was after, and I hadn't realised the importance of the condenser capacity. Why the lower of the two calculated temperatures defines the condensing temperature I cannot understand, but still, I have my answer.

  11. #11
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    The condensing temperature determines the head pressure. The hotter the cooling medium the higher the condensing temperature and therefore head pressure and vice versa. The compressor's job is to raise the low temp low pressure gas from the evaporator to a high temp high pressure gas in the condenser.

  12. #12
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    Quote Originally Posted by gateeight View Post
    icemeister, thank you very much for your detailed response. Your final statement is exactly the answer I was after, and I hadn't realised the importance of the condenser capacity. Why the lower of the two calculated temperatures defines the condensing temperature I cannot understand, but still, I have my answer.
    Heat flows from hot to cold...and it also tends toward the lowest temperature.

    If you were to consider an air-cooled condenser with a TD of 30F TD and let's say 100F ambient with the same 15,000 Btuh THR, the expected condensing temperature would be 130F. Now if you add the water-cooled condenser with its 20F TD and have 70F water entering, you would see a 90F condensing temperature. Since this is lower than the ambient air, there would be no condensing at all in the air-cooled condenser. Actually, It would only be doing some desuperheating, but no condensing.

    Clear as mud?

  13. #13
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    Quote Originally Posted by ryan1088 View Post
    Is this just a theoretical question or what are we trying to get to? Why on earth would you want to do that?
    I'm a bit puzzled, too.

    It would depend on the sizing of the coils and the temperature of the mediums involved.

    Now, if I'm reading between the lines well enough, I'm envisioning a water heater type setup and, as was previously mentioned, this isn't uncommon.

    You'll want the water portion first, though, to take advantage of the superheated discharge gas rather than wasting that heat to the air.

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